American Michael Sharp and sweedish Zaida Catalan, were found in a shallow grave in the DRC on Monday

Michael Sharp, a 34-year-old from Kansas, and Zaida Catalan, 36, from Sweden, went missing on March 12, having set out to investigate mass graves in the Kasai province.

On Monday their bodies were found in a shallow grave, along with that of Bete Tshintela.

On Wednesday the state department updated its previous advice, and now recommends that all US citizens avoid unnecessary travel to the DRC.

Authorities in the country have vowed to investigate the murders, and Sweden has opened an investigation.

“Zaida Catalan worked tirelessly for peace and justice, and risked her own life to save others,” said Stefan Lofven, the Swedish prime minister. “Sweden is naturally ready to assist in this work.”

Antonio Guterres, the UN secretary-general, paid tribute to the pair.

“Michael and Zaida lost their lives seeking to understand the causes of conflict and insecurity in the DRC in order to help bring peace to the country and its people,” he said. “We will honour their memory by continuing to support the invaluable work of the Group of Experts and the whole UN family in the DRC.”

Nikki Haley, the US ambassador to the UN, also praised Sharp’s professionalism.

“Michael was working on the front lines of what we try to do at the United Nations every day: find problems and fix them,” she said in a statement. “He selflessly put himself in harm’s way to try to make a difference in the lives of the Congolese people.”

His father, John Sharp, told NBC News that an “unidentified militia group” was responsible for his kidnapping when he first learned his son went missing. Mr Sharp said his son was dedicated to doing humanitarian work in eastern Congo even before he joined the United Nations.

Their disappearance marked the first time UN experts had been reported missing in Congo, Human Rights Watch reported, and was the first recorded disappearance of international workers in the Kasai provinces.

The UN last week reported that more than two dozen mass graves have been discovered since January in three Kasai provinces. Five videos have emerged in recent weeks that appear to show Congolese soldiers firing on militia members.

While the violence is linked to local power struggles, there are also clear ties to Congo’s current political crisis, according to Human Rights Watch.

Anger has been growing in the country at long-delayed presidential elections, and dozens were killed in December amid protests as President Joseph Kabila stayed on past the end of his mandate. A deal reached between the ruling party and opposition to hold elections by the end of this year, without Mr Kabila, remains fragile as the UN urges its implementation.